Spinal muscular atrophy (SMA) tends to be identified early in babies who have noticeably and unusually weak muscles, but diagnosing this neuromuscular disorder later in life can be more of a challenge. Now studies suggest that when muscle weakness develops in children, adolescents, and young adults, ultrasound may help pinpoint SMA sooner for faster diagnosis and treatment.
SMA is most often diagnosed in newborns and infants through genetic testing, but ultrasound may play a role in diagnosis or assessment depending on a person’s age and severity of symptoms. Researchers have found that ultrasound can help evaluate muscle deterioration in neuromuscular diseases such as SMA and Duchenne muscular dystrophy. A 2010 study showed that ultrasound can help doctors tell the difference between subtypes of SMA. More recently, results of a 2023 study revealed that ultrasound imaging of the limbs and respiratory muscles can help assess disease progression in all four types of SMA.
Read on to learn more about how ultrasound can be used in assessing SMA, which particular sign doctors look for in scans, and how ultrasound may help you get the right diagnosis.
Ultrasound, also called a sonogram or ultrasonography, is a tool that uses sound waves to produce images of organs and structures in the body. These images often give insight into diagnosing a condition or assessing disease severity or disease progression. When it’s an option for testing, ultrasound is a popular choice because it:
In the case of SMA, ultrasound can be used to look at multiple muscle groups in a short amount of time.
Ultrasound isn’t generally used to diagnose spinal muscular atrophy type 1, the most common form. Type 1 SMA is usually a straightforward diagnosis based on the infant’s behavior — such as being unable to sit by themselves, swallow well, or cry loudly — and a physical examination. Prenatal testing is available, but ultrasound is not used to detect SMA before a baby is born.
Later, ultrasound can play a role in the other subtypes of SMA, classified as types 2, 3, and 4. Diagnosing these subtypes can be trickier because they come on later in life and vary in severity. The symptoms are less obvious, so getting the right diagnosis can be challenging. Usually, on follow-up, your doctor will order a variety of neurological tests, which may include an ultrasound to look for a possible warning sign of SMA.
If you have new muscle weakness, your doctor might perform an ultrasound to look for fasciculations — quick, involuntary muscle contractions or twitches. Fasciculations, which aren’t painful, can occur in SMA and show up on an ultrasound image.
In a small 2022 study of 10 adolescents and adults with SMA and nine people with different genetic muscle disorders, ultrasound images showed fasciculations in all participants with SMA but just three people with other muscle diseases. However, even though fasciculations were seen in everyone with SMA, the study authors noted that genetic testing remains the gold standard and the most accurate way to properly diagnose SMA.
An earlier study that included 18 children with SMA found that 16 had muscle changes that could be detected on ultrasound. Two others, who were infants, had normal ultrasound images. The researchers suggested that the two infants were too young for changes in muscles — the hallmark of SMA — to have yet occurred. As such, ultrasound is typically used only when late-onset SMA is suspected.
Diagnosing muscle wasting or loss of muscle strength in children and adults can be challenging. Although ultrasound doesn’t yet replace genetic testing for a definitive diagnosis of late-onset SMA, it can help narrow down the possible conditions. If someone has muscle weakness of unknown cause, their doctor will likely order a variety of tests, which may include:
An ultrasound of the muscles may be part of that battery of tests. If no muscle fasciculations show up on the ultrasound images, SMA may be the less likely culprit, based on current research findings. If fasciculations are seen, then genetic testing is the most likely next step to determine if SMA is causing the lost muscle strength.
Ultimately, ultrasound may be a useful additional tool in properly diagnosing late-onset SMA and other neuromuscular disorders that are hard to narrow down. Getting an ultrasound may lead to a more rapid diagnosis and earlier treatment. If you or your loved one has unexplained muscle weakness or wasting, or if you are experiencing more and more difficulty with motor function but don’t have a diagnosis, talk to your doctor about the role of muscle ultrasound.
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Has a doctor ever used ultrasound to evaluate your muscles? Did it lead to a diagnosis of SMA? Share your experience in the comments below, or start a conversation by posting on your Activities page.